The spatially distributed iris features make them ideal for video-based tracking, a method employed in eye-tracking devices to track a subject's gaze. 4 It also contains many variations in pigments caused by differences in melanin types, dark pigment spots, 5 bright collagen spots (Wolfflin nodules 6), thin-layered gaps (Fuchs crypts 7), and sometimes brownish pigment spots surrounding the limbus (conjunctival melanosis 8). Sometimes the iris contains circular contraction furrows in the outer region caused by wrinkling due to changes in pupil size. The numerous ridges demark blood vessels that extend radially across both regions. The more thickly layered collaret lies between the inner and outer regions where the underlying constrictor and radial muscles touch. 1 – 3 Variations in the spatial layout and colors are mostly distributed across two regions: an inner, pupillary region that extends to the collaret and an outer, ciliary region that extends to the sclera border, also called the limbus. It is rich in texture, containing complex features that constitute numerous individual landmarks and variations in appearance across the eyes.

The iris is an anatomically distinct and, one may say, rather magnificent piece of the eye. We expect that irissometry may pave the way to novel eye trackers and diagnostic tools in ophthalmology. Tracking features in the iris produce more robust eye position signals. This elastic property explains the instability of the pupil border and the related position errors induced by eye movement and pupil size in pupil-based eye-tracking. The iris regions near the pupil appear to be more elastic than the outer regions near the sclera. Importantly, iris-based eye position detection led to more stable signals than pupil-based detection. The pattern of iris densities across eccentricities and pupil size showed highly similar patterns across participants, highlighting the robustness of this elastic property. The time analysis of densities showed that the inner regions of the iris stretched more strongly as compared with the outer regions of the iris during pupil constrictions. We tracked changes in iris surface structure between the pupil and sclera border (limbus) by calculating local densities (distance between feature points) across evenly spaced annular iris regions.

To evoke pupillary responses, we repeatedly presented visual and auditory stimuli to healthy participants while we additionally recorded their right eye with a macro lens–equipped camera. Here, we report an application of a multi-feature iris tracking method, which we call irissometry, to investigate how the iris deforms and affects the eye position signal as a function of pupil size. It is unclear how the iris deforms during changes in pupil size.

Split pupil